Question

A ball is thrown vertically upward attains a Max. Height of 45m.the time after velocity of the ball become equal to half the velocity of projection,??

Answer 1

initial velocity = u
final velocity = v= 0
v² + u² = 2gh
or 
u = √2gh
u = 30 m/s
by the 1st eq of motion
v = u - gt
15= 30 - 10t
t = 1.5 s